The determination of genera being mainly an artificial mode
of aiding research, or conveniently gronping together forms
possessing in common important and obvious characteristics of
structure or of function, must, to a great extent, be influenced
by the theoretic views, and be left to the experience or judgement
of the individual inquirer ; but it is far otherwise with the
determination of species. We here seek to discover the distinctions
which have been impressed by nature upon every
individual derived by reproduction or by self-division from the
original product of the creative act. Such distinctions may, to
our powers of apprehension or discovery, be of the slightest
kind, and may, in the marvellous minuteness and multiplicity of
these organisms, blend into one another by gradations too fine
to be detected by any appliances that we can employ. In an
attempt to accomplish this object, it is a point of the first
importance to select peculiarities of structure or organization
which in their main features are common to every species in
each genus we may adopt, and yet exhibit variations in each
species of such genus. If several such peculiarities can be
found, our task would be an easy one ; but the simplicity of
organization in the Diatomaceæ forbids this expectation, and
usually reduces us within narrower limits. I shall lay before
the inquirer the results of my own experience on this subject,
without claiming for the conclusions to which I have arrived
any absolute authority, feeling assured that a far wider knowledge
of these forms than that to which I can at present pretend,
would be necessary to give to such conclusions the weight of
established truth.
We have seen that the ordinary Diatomaceous frustule owes
its reproduction to the protoplasmic contents of the sporangial
frustule formed during the process of conjugation.
The embryonic frustules which are generated within the
sporangial cyst, having, by their increase in size, burst the membrane
which contains them, escape from the cyst, and in a
definite, but unascertained period, reach the mature form and
size of the ordinary frustule.
The further growth and modification in form of the individual
cells seem now to be arrested by the consolidation of the sili-
IN T R O D U C T IO N .
ccous valves ; and the multiplication of these forms, of the size
thus reached, goes on with inconceivable rapidity by means of
self-division.
The size of the mature frustule before self-division commences,
is, however, dependent upon the idiosyncrasy of the
embryo, or upon the circumstances in which its growth takes
place ; consequently a very great diversity in their relative magnitudes
may be noticed in any large aggregation of individuals,
or in the same species gathered in different localities. Moreover,
while a typical outline of its frustule is the general characteristic
of a species, this outline may be modified by the
accidental circumstances which surround the embryo during its
growth, and the development of its siliceous epiderm.
The process of self-division now intervening, and necessarily
stereotyping the shape with which it commences, multitudes of
frustules slightly deviating from the normal form are subsequently
produced, so that the observer, judging from a single
gathering, may be led to fix upon a variety as representing the
typical form and size of the species.
It follows from these circumstances that neither size nor outline
is sufficient to enable the observer under ordinary circumstances
to determine the species of a Diatomaceous frustule:
well-marked diversities of shape, depending upon structural
peculiarities, may be an auxiliary guide in the adoption of
generic distinctions ; but the slighter and accidental variations,
which occur dm'ing the development of the individual frastules,
forbid us to employ such characters in the determination of
specific forms. If the observer have the means of comparing
specimens in sufficient numbers and fr’om various localities, he
may fix with tolerable accuracy upon the magnitude and form
which may be regarded as the average and type of the species ;
but without these opportunities, a reliance upon such characters
wül lead to an undue multiphcation of species, and to a confusion
and indefiniteness in their arrangement that will embarrass
and mislead the future inquirer.
One specialty in the Diatomaceous frustule, presenting important
modifications that may be introduced with propriety
into our generic descriptions, and at the same time offering